Resonators are used as the main structural part in the manufacture of oscillators and filters. The important characteristics of resonators include, for example: (Q-value), size, tunability, tendency to oscillate at the harmonic frequencies, mechanical stability, temperature and humidity stability and manufacturing costs.
The resonator constructions that are known so far include the following:
1) Resonators compiled of discrete components, such as capacitors and inductors
Resonators of this kind entail the drawback of internal dissipation of the components and therefore clearly lower Q-values compared to the other types.
2) Microstrip resonators
A microstrip resonator is formed in the conductor areas on the surface of a circuit board, for example. The drawback is radiation dissipation caused by the open construction and thus relatively low Q-values.
3) Transmission line resonators
In a transmission line resonator, the oscillator consists of a certain length of a transmission line of a suitable type. When a twin cable or coaxial cable is used, the drawback is relatively high dissipation and a relatively poor stability. When a waveguide is used, stability can be improved, but the dissipation is still relatively high because of radiation when the end of the pipe is open. The construction can also be unpractical large. A closed, relatively short waveguide resonator is regarded as a cavity resonator, which is dealt with later.
4) Coaxial resonators
Resonators of this type have a construction which is not merely a piece of coaxial cable but a unit which was originally intended as a resonator. FIG. 1 shows a coaxial resonator. It includes, among other things, an inner conductor 101 and an outer conductor 102, which are air-insulated from each other, and a conductive cover 103, which is connected with the outer conductor. A relatively good result can be achieved by this construction. The length of the resonator 1 is at least one fourth, .lambda./4, of the wavelength of the variable field effective in it, which is a drawback when aiming at minimizing the size. The width can be reduced by reducing the sides D of the outer conductor and the diameter d of the inner conductors. However, this leads to an increase of resistive dissipation. In addition, because of the reduction in the thickness of the construction, it may be necessary to support the inner conductor by a piece 107 made of a dielectric material, which causes considerable extra dissipation in the form of dielectric loss and increases the manufacturing costs. Furthermore, a drawback of the known coaxial resonators is a tendency to oscillate at the third harmonic of the basic frequency. This extra component (spectrum when the signal is transferred) is so strong that it must be removed by a separate filter.
A filter comprising coaxial resonators is disclosed in Swiss Patent No 532 864, having a capacitive coupling between adjacent coaxial resonators. The capacitive coupling between the resonators may be adjusted.
An other filter comprising coaxial resonators is disclosed in Japan publication JP 60090402, each resonator having an arm extending in the propagation direction of a transmit signal to increase the capacitive coupling between the inner conductor and the outer conductor.
5) Helix-resonators
This type is a modification of a coaxial resonator, in which the cylindrical inner conductor is replaced by a helical conductor. Thus the size of the resonator is reduced, but the clearly increased dissipation is a drawback. Dissipation is due to the generally small wire diameter of the inner conductor.
6) Cavity resonators
Resonators of this type are hollow pieces made of a conductive material, in which electromagnetic oscillation can be excited. The resonator can be rectangular, cylindrical or spherical in shape. Very low dissipation can be achieved with cavity resonators. However, their size is a drawback when the aim is to minimize the size of the construction. In addition, the tunability of most cavity resonators is poor.
7) Dielectric resonators
Coaxial cables or a closed conducting surface is formed on the surface of the dielectric piece. The advantage is that the construction can be made in a small size. Relatively low dissipation can also be achieved. On the other hand, dielectric resonators have the drawback of relatively high manufacturing costs.
8) Hat resonators
A subclass of coaxial resonators, here called hat resonators, are described in U.S. Pat. No. 4,292,610 by Makimoto. This type of resonators is a cavity resonator, as described above, with an additional disc on the open end of the waveguide, having a larger diameter than the waveguide. An advantage is that the resonator can be made compact. Relatively low dissipation can also be achieved. The surface of the disc and distances to the walls of the resonator are dimensioned so that due to extra capacitance created between the disc and the cavity, the resonator can be made substantially smaller compared to one without the additional disc.